The hot exhaust gases generated in glass melting contain nitrogen oxides (NOx) to a great extent in addition to CO2, O2, H2O and N2. Nitrogen oxides must be removed from the exhaust gas because they are environmental toxins. This NOx cleaning may be complex under some conditions because multiple pollutant components are present in the exhaust gas and would act as so-called catalyst poisons to interfere with NOx cleaning and therefore must be mostly removed from the exhaust gas before the NOx cleaning begins. Typical pollutant components which may be present in the exhaust gases from a glass melting tank include SO2, HCl, heavy metals, in particular Hg, dioxins, furans and condensable residues as well as dusts. The boron and/or arsenic compounds, which are often present in the exhaust gas, are especially problematical for cleaning exhaust gas from glass melting tanks, but other problematical substances include calcium, strontium and barium compounds and/or other constituents originating from additives to the SiO2-based raw material for making glass. The corresponding pollutant components occur in a wide variety of types of glass, in particular those that are used for LCD screens. In many cases, compounds containing arsenic are added to a glass melt to make the melt as liquid and free of bubbles as possible. The boron nitrate added to many types of glass results in a high NOx content in the exhaust gas. Other additives that evaporate as such or in a different form from the glass melt usually also include pollutants which must be removed from the process exhaust gas (exhaust gas from combustion and from the glass melting process).
In the past, exhaust gas at approximately 1650° C. would be cooled greatly, especially by diluting it with air and/or by quenching to yield temperatures suitable for use of a bag filter as a precleaning stage, for exhaust gas cleaning of the glass furnace exhaust gases. The boron and arsenic compounds present in the exhaust gas go directly from the gas phase to the solid phase at temperatures between 85° C. and 100° C. When the exhaust gas is cooled, complete sublimation of the aforementioned pollutant components is not achieved. The degree of cleaning that can be achieved with a bag or cloth filter depends on the particle size. This in turn depends on the temperature, the gas pressure and the gas dwell time in the evaporative cooler (quenching). The lowest possible temperature and a relatively long dwell time would thus have to be selected to achieve a sufficient particle size so that then the particles could be separated by a bag or cloth filter. Whether boron and arsenic compounds already undergo sublimation in the evaporative cooler at the temperatures occurring on the bag filter, the crystals thus formed are very small. Furthermore, the formation of crystals, as mentioned above, requires a certain dwell time in the sublimation temperature range. Most boron and arsenic compounds are therefore not adequately removed in the bag filter, so a subsequent wet scrubbing has been necessary to obtain a degree of precleaning that will allow a subsequent NOx cleaning without excessively great damage to the catalyst. However, wet scrubbing leads to further environmental problems because the washing liquid in turn must be freed of the components contained therein before the washing liquid is reused or removed from the process as wastewater. The gas cleaning problem has thus been shifted to a wastewater purification problem.
A large number of methods that can be used commercially such as SCR, SNCR and scrubbing with oxidizing agents are known for reducing the NOx content. Apart from scrubbing with oxidizing agents, all the methods used commercially so far operate above a minimum exhaust gas temperature, which is at least 160° C. (low-temperature catalysts) and is generally above 300° C. For the most popular technology using catalysts, the pollutant components mentioned above constitute catalyst poisons to at least some extent.